A proton exchange polymer membrane fuel cell (PEMFC) and a direct methanol fuel cell (DMFC) based on a proton conductive polymer electrolyte membrane are energy generation systems having high energy efficiency and a low emission of air polluting substances. Particularly, unlike petroleum which is limited recoverable reserves, these fuel cells are advantageous in that the use of hydrogen or methanol as a fuel is almost perpetual. Thus, the fuel cells have been studied with great interest as an alternative energy source.
FIG. 1 is a basic schematic view of a membrane-electrode assembly (MEA) constituting a fuel cell for producing electric energy and water/heat.
With reference to FIG. 1, A proton conductive polymer electrolyte membrane 1 generates protons, as in the following reaction, through a catalytic reaction of hydrogen/methanol 4 provided to an anode with an anode catalyst 9.
(In the case of PEMFC)2H2→4H++4e−
(In the case of DMFC)CH3OH+H2O→CO2+6H++6e−
Thusly formed protons move to a cathode via the proton conductive polymer electrolyte membrane such as a proton transport 7. And, electrons moved through an external circuit 12 and air or oxygen 5 provided to the cathode meets to produce water, electric energy and heat 6 by a reduction reaction as in the following reaction.
(In the case of PEMFC)O2+4H++4e−→2H2O
(In the case of DMFC)3/2O2+6H++6e−→CO2+2H2O
At this time, the electrode portion where redox reaction occurs has a structure in which metal nanoparticles such as platinum (Pt) and ruthenium (Ru) 9 or 10 are deposited on graphites 11 to form a catalytic particle mass, and the catalytic particle mass is mixed with a catalyst binder 8, which is a proton conductive polymer substance. The electrochemical performance is deeply related to formation of a suitable triple phase boundary within the electrolyte, electrode, and fuel.
In general, as a catalyst binder for preparing an electrode, a Nafion binder (EW=1,100) dispersed in a mixed solution of isopropyl alcohol and water is used due to its high proton conductivity and chemical stability.
Unlike using the Nafion membrane as a polymer electrolyte membrane, when using a hydrocarbon polymer such as sulfonated polysulfone, poly(arylene ether sulfone), poly(arylene ether ketone), polyimides, or polyphosphazene, which is currently preferred as a substitute for the polymer electrolyte membrane, there is a big difference in the compatibility with Nafion as a catalyst binder. Thus it has high interfacial resistance between the polymer electrolyte membrane and the catalyst layers and forms inappropriate triple phase boundary, thereby the polymer membrane has low electrochemical unit cell performance. In addition, by repeating the humid/dry condition according to the operation of the fuel cell, delamination of the electrode layers from the electrolyte membrane in MEA is occurred, and as a result, the fuel cell performance is rapidly deteriorated. Therefore, in order to resolve these serious interfacial problems, several attempts have been conducted.
For example, attempts to reduce interfacial resistance between a polymer electrolyte membrane and catalyst electrodes using the same polymeric material for the membrane and electrodes are introduced in Journal of Power Sources 163 (2006) 56-59, Electrochinica Acta 52 (2007) 4916-4921, Journal of Power Sources 169 (2007) 271-275, Journal of Power Sources 170 (2007) 275-280, and the like. However, despite the improvement in the adhesiveness between the electrolyte membrane and the catalyst layer, these attempts are still restricted in their use, because of the problems such as limits to dissolution of the polymer used as a catalyst binder, significantly reduced catalytic activity due to inappropriate solvent selection, low electrochemical unit cell performance in spite of high proton conductivity, and low chemical/electrochemical stability.
There also have been attempts to introduce fluorine groups to a sulfonated polymer electrolyte membrane to improve interfacial characteristics (J. Electrochem. Soc. 151 (2004) A2150-A2156; Electrochim. Acta 49 (2004) 2315-2323; Journal of Membrane Science 281 (2006) 111-120, Polymer 47 (2006) 808-816; Electrochimica Acta 51 (2006) 6051-6059, Polymer 47 (2006) 4123-4139; Journal of Membrane Science 294 (2007) 22-29; Journal of Membrane Science 299 (2007) 8-18).
Generally, a method for preparing a hydrocarbon polymer by a condensation reaction using partial-fluorinated monomers is used. However, in this case, there is a problem that the condensation reaction is interfered with the high electronegativity of fluorine, and as a result, it has a limitation of obtaining hydrocarbon polymer having a high molecular weight. In addition to the molecular weight reduction problem, there are problems of degradation of the polymer chains and difficulty in controlling a degree of sulfonation through a post-sulfonation process (Electrochimica Acta 49 (2004) 2315-2323).
To this point, several examples for substituting fluorine groups to the polymer by direct fluorination (Orfanofluorine Chemistry: Principles and Commercial Applications, Plenum Press, New York, 1994, p. 469, Journal of Fluorine Chemistry 128 (2007) 378-391) have been mentioned. However, there have yet been attempts to conduct direct fluorination on a proton conductive polymer electrolyte membrane for fuel cell.
Korean Patent Laid-open Publication No. 2007-98325 discloses a method for fluorinating a hydrophobic region of a proton conductive block copolymer not including sulfonic acid groups by swelling in a hydrocarbon solvent (C1-based solvent) and subjecting to a Friedel-Craft reaction using a fluorinating agent which dissolves in the same solvent. As a result, surface modification in the hydrophobic region of a grafted structure containing a fluoro-compound is exhibited. In this case, a catalyst such as SnCl4, FeCl3, or AlCl3 must be used to facilitate the reaction.
However, the fluorination method according to above-mentioned patent has problems that additional costs for using a solvent, treatment after the use of the solvent and a catalyst are generated. Moreover, since the fluorination is carried out in the hydrophobic region of the polymer, there are barely any effects on the spontaneous dissociation of the sulfonic acid groups adjacent to the fluorine groups due to high electronegativity of the fluorinated polymer. As a result, the fluorination has barely any influence on improvement in the proton conductivity of the polymer membrane. Furthermore, there are problems that a great amount of time is required in drying after swelling and reacting the polymer in the solvent and the fluorination process becomes more complicated.